Hi Jo;
The Amber amp's output were not actually paralleled, the output of one was fed to the input of the other, and reduced by resistors. Why not just parallel the inputs? The switch is Mono/stereo. The amp was not designed to be bridged. Seems to me to be a bad idea to use the speaker level signal of one channel, reduced by resistors and re-amplified by the other channel. Wouldn't S/N ratio and distortion be worse?
The Amber amp's output were not actually paralleled, the output of one was fed to the input of the other, and reduced by resistors. Why not just parallel the inputs? The switch is Mono/stereo. The amp was not designed to be bridged. Seems to me to be a bad idea to use the speaker level signal of one channel, reduced by resistors and re-amplified by the other channel. Wouldn't S/N ratio and distortion be worse?
because paralleling the inputs only produces the same 35 watts out as the amp had already, on two speakers. Doubling the voltage out on one speaker doubles the wattage on one speaker. This is "bridging", a popular practice in the bar band world and a feature that would make a few more sales. bands don't want to carry around a number of 35 W speakers. One big thousand watt amp would be more to bar band liking, but not available in 1972. The bar band market now is dominated by "inuke" 6000 w amps, that have a watt rating higher than the wall plug they are connected to. Smoke & Mirrors will always sell better than boring old reality. The established bands that have been around the block a few times, when they buy a 6000 W amp, they make sure it is powered by 480 vac 3 phase 30 amp feed.
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There are no suitable substitutes for the 391 that I’m aware of. You could try to duplicate the chip’s internal circuitry with discrete components using the Equivalent Schematic and Connection Diagram in the datasheet for guidance but I’m not aware of anybody doing that and I don’t know what new set of problems you might create. Would be an interesting project, though!
Bias should be set at 150mA per the procedure outlined in the service bulletin (Peter S attached it above). Make sure you let the amp truly achieve thermal equilibrium and verify the bias readings one last time before you screw the lid back on. Good luck!
-Mark S.
Actually, the voltage output of the input channel is fed into the INVERTING input of the other channel, which, because of the two 100K resistors in the output/feedback loop, is set up as a unity gain current amplifier. As a result, the two channels effectively act as a bridged push-pull mono amplifier (note that this setup is right out of the LM391 data sheet). It’s no different than any other bridging circuit which uses extra circuitry to invert the signal to one channel to provide push-pull operation. However in the Amber implementation, the inverting is done without extra active circuitry by using the inverting input of the other channel as described above. With one channel “pushing” while the other amp “pulls”, and the load connected between the two channels, twice the voltage is available and therefore twice the current. With an 8 ohm load, each channel effectively acts as if it’s “seeing” a 4 ohm load, and since the amp is rated at 100W into 4 ohms per channel, the bridged mono rating is 100x2 = 200W. Hope this makes sense!
-Mark S.
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Thanks Mark and Jo;
I was wrong, I thought it was just stereo or dual mono, but I see now, the left output is fed into the - input on the R chan lm391 and the output taken from the red speaker terminals....so bridged...Sorry Jo, you tried to tell me!
So all set to return it to my friend when I noticed a low level hum...but only when both inputs are connected. Dead quiet with nothing connected to the inputs. I then connected a short RCA cord from left in to right in---hum returns
I was wrong, I thought it was just stereo or dual mono, but I see now, the left output is fed into the - input on the R chan lm391 and the output taken from the red speaker terminals....so bridged...Sorry Jo, you tried to tell me!
So all set to return it to my friend when I noticed a low level hum...but only when both inputs are connected. Dead quiet with nothing connected to the inputs. I then connected a short RCA cord from left in to right in---hum returns
Hum elimination doesn't communicate well through schematic diagrams and certainly not sample circuits in IC datasheets. Details of the circuit, especially the grounding/return side matter. Something involving a ground loop including the shield of the cable you connected to the two inputs, matters. I'd look at AC sources in that ground loop, probably including the chassis at al. Is is one side or the other side humming, or both? Is there a way for current to flow between inputs that includes the transformer? Is the transformer shielded in it's own steel case? is there a copper band around it? Does grounding of a transformer case include rust resistant star washers to insure the currents go to AC cord 3rd pin?
I just re-activated a scope to go on hum patrol in my ST120/AX6 chassis, without luck so far. I had it nearly hum free for 5 years, then a black (return) wire popped off the AX6 board. I soldered it back some other black wires, now it hums. What did I do wrong?
Good luck.
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Thanks Ivo; I can't seem to find R24 on the boards. The boards I have seem to have many differences from the schematic My boards do not have a zener regulating the IC voltage. Also, my boards have a connecting to pin 4 on the IC (data sheet calls pis 4 "RIPPLE C"). Pin 4 is not shown on the schematic.....this seems relevent.
Attached is the data sheet for the LM391, BTW, those 'off shore' chips are still holding up.
Attached is the data sheet for the LM391, BTW, those 'off shore' chips are still holding up.
One power supply and 2 amplifiers (left / right) is the recipe for the ground loop, if not enough attention is paid to the circulating currents (signal, power, HF, etc.). One possible simple solution is the mysterious R24...it breaks the ground loop between the left and right channels, so take a good look at your pcb's and see if there is no accidentally installed jumper instead of R24 that "breaks" the connection between the signal and all other grounds in the amplifier.
The explanation for Cr ripple filter capacitor is given on page 4 datasheet, my advice is to use it ...
The explanation for Cr ripple filter capacitor is given on page 4 datasheet, my advice is to use it ...
Mark:
It is gratifying that you have worked on the team that brought to light what some of us consider to be a sonic gem, and I wish you would share some of your knowledge with those we have kept working and improving the sound of these, for several decades now.
I have a couple of them with the heatsinks on the sides, which came with 470uF capacitors which I have replaced with a 1000uF / 50V Nichicon Muse bypassed by 0.1uF MKT, just like the other caps on the PCBs. The resistors (all of them) were replaced by metal film with exact values, but of 1 / 2w, and with better tolerances.
Do not forget that all the original components have already worked for more than 4 decades.
I am very satisfied with the sound my 2 Amber have today, they sound much better than when new.
Would you be so kind as to answer the following?
1. How many watts is the transformer used?
2. On the NS sheet for the LM391-N100, they suggest 10uF for the feedback capacitor, while mine came with 47uF.
Is there a technical reason why the designers used a higher value for this capacitor?
3. My units came without R8, but here it has been said that there are versions with R24 (…?)
4. Are there other modifications that you can recommend?
Regards,
It is gratifying that you have worked on the team that brought to light what some of us consider to be a sonic gem, and I wish you would share some of your knowledge with those we have kept working and improving the sound of these, for several decades now.
I have a couple of them with the heatsinks on the sides, which came with 470uF capacitors which I have replaced with a 1000uF / 50V Nichicon Muse bypassed by 0.1uF MKT, just like the other caps on the PCBs. The resistors (all of them) were replaced by metal film with exact values, but of 1 / 2w, and with better tolerances.
Do not forget that all the original components have already worked for more than 4 decades.
I am very satisfied with the sound my 2 Amber have today, they sound much better than when new.
Would you be so kind as to answer the following?
1. How many watts is the transformer used?
2. On the NS sheet for the LM391-N100, they suggest 10uF for the feedback capacitor, while mine came with 47uF.
Is there a technical reason why the designers used a higher value for this capacitor?
3. My units came without R8, but here it has been said that there are versions with R24 (…?)
4. Are there other modifications that you can recommend?
Regards,
Oldeng:
It seems that your Amber belongs to the last units manufactured, the same ones that show important differences with the pair bought by me in l982, which have only 2 capacitors, 470uF (on each PCB) with each 68 Ohm series resistors, that feed the + and -B of the IC.
But, you are talking about 2 capacitors, 1,000uF on each PCB, which being in parallel add up to 2,000 uF for each one, which is a very significant change with respect to mine, in which their bypasses are 0.1uF / 200v , instead of the 0.022uF that you indicate.
Also, the only non-polarized aluminum capacitor on each of my PCBs is 47uF / 16v (connected to ground) which together with the 5k and 100K resistors coming out of pin 9 of LM391-N100, make up the feedback circuit. . Also, and only for use in stereo mode, there is a single blocking capacitor, 1uF, which in my case I have replaced with an MKT of the same value.
As far as I know, no Amber Series 70 is inverting.
Would you be so kind to publish a PDF with the circuit that currently has your version, to verify the modifications that the designers made to units like yours?
Does your unit have: R24, where is it and what is it for?
Regards,
It seems that your Amber belongs to the last units manufactured, the same ones that show important differences with the pair bought by me in l982, which have only 2 capacitors, 470uF (on each PCB) with each 68 Ohm series resistors, that feed the + and -B of the IC.
But, you are talking about 2 capacitors, 1,000uF on each PCB, which being in parallel add up to 2,000 uF for each one, which is a very significant change with respect to mine, in which their bypasses are 0.1uF / 200v , instead of the 0.022uF that you indicate.
Also, the only non-polarized aluminum capacitor on each of my PCBs is 47uF / 16v (connected to ground) which together with the 5k and 100K resistors coming out of pin 9 of LM391-N100, make up the feedback circuit. . Also, and only for use in stereo mode, there is a single blocking capacitor, 1uF, which in my case I have replaced with an MKT of the same value.
As far as I know, no Amber Series 70 is inverting.
Would you be so kind to publish a PDF with the circuit that currently has your version, to verify the modifications that the designers made to units like yours?
Does your unit have: R24, where is it and what is it for?
Regards,
1. Can't help on the transformer specs...I've long since lost any detailed specification paperwork pertaining to individual components.
2. I believe the larger cap was used to open up the bandwidth somewhat, but that design update occurred before my time so can't speak to it directly.
3. Resistor numbers changed several times over the years as circuit mods were incorporated and schematics subsequently updated, so not sure what happened to your R8. Likewise with R24...most of these changes were either reliability improvements or chasing further reductions in hum and noise as the PCB and chassis layout evolved over the years.
4. As far as additional mods, I prefer to say true to the basic design and only replace or improve parts where specifically warranted due to age and/or not meeting specification.
Sorry for the late reply...hope this helps!
-Mark
swierczekml:
I totally agree with what was expressed in number 4 of your answer.
I do not agree with arbitrary changes in the original circuit, but I do agree with improving the quality of the components that did not exist at the date of manufacture.
My 2 units today have 2 Nichicon Muse capacitors of 1,000Uf / 50V, per channel (the 64,000uF of the power supply allows this value and even much more, I think) ALL 0.1Uf capacitors were replaced by 0.1Uf MKT, C11 was replaced by 1Uf MKT, R5 was replaced by a Dale of the same value, but with less tolerance. R7 and R12 were replaced by .22 Ohm/1w metal film. at 2%. The bias was adjusted to 150 mA. The power supply voltage is between 41 and 42 vDC, with 120vAC.
My units did not even come with R8, worse with R24.
My 2 Amber that originally came with standard 470 capacitors, whose response in the bass was shy for my taste, now with the Muses, the bass is firmer and deeper, in the treble, there has been no major difference. This is the reality.
I have seen in the network, Ambers with 2,220 uF to feed the IC, but I do not know if these would imply other changes in the circuit.
Thanks for your reply.
MAQM
I totally agree with what was expressed in number 4 of your answer.
I do not agree with arbitrary changes in the original circuit, but I do agree with improving the quality of the components that did not exist at the date of manufacture.
My 2 units today have 2 Nichicon Muse capacitors of 1,000Uf / 50V, per channel (the 64,000uF of the power supply allows this value and even much more, I think) ALL 0.1Uf capacitors were replaced by 0.1Uf MKT, C11 was replaced by 1Uf MKT, R5 was replaced by a Dale of the same value, but with less tolerance. R7 and R12 were replaced by .22 Ohm/1w metal film. at 2%. The bias was adjusted to 150 mA. The power supply voltage is between 41 and 42 vDC, with 120vAC.
My units did not even come with R8, worse with R24.
My 2 Amber that originally came with standard 470 capacitors, whose response in the bass was shy for my taste, now with the Muses, the bass is firmer and deeper, in the treble, there has been no major difference. This is the reality.
I have seen in the network, Ambers with 2,220 uF to feed the IC, but I do not know if these would imply other changes in the circuit.
Thanks for your reply.
MAQM